1. Field of the Invention
The present invention relates to disk drives. In particular, the present invention relates to servo writing substantially linear servo wedges to reduce the overwrite effect in perpendicular magnetic recording.
2. Description of the Prior Art
When manufacturing a disk drive, servo sectors 20–2N are written to a disk 4 which define a plurality of radially-spaced, concentric data tracks 6 as shown in the prior art disk format of FIG. 1. Each data track 6 is partitioned into a plurality of data sectors wherein the servo sectors 20–2N are considered “embedded” in the data sectors. Each servo sector (e.g., servo sector 24) comprises a preamble 8 for synchronizing gain control and timing recovery, a sync mark 10 for synchronizing to a servo data field 12 comprising coarse head positioning information such as a Gray coded track addresses for use during seeking, and servo bursts 14 which provide fine head positioning information for use during tracking. In a “headerless” format, the servo data field 12 also includes information identifying the data sectors between the servo sectors.
The servo sectors 20–2N are typically written to the disk 4 using the head 16 internal to the disk drive, for example using an external servo writer or using a self-servo writing technique. An external servo writer inserts a positioning pin into the head disk assembly (HDA) of the disk drive to actuate an actuator arm 18 about a pivot 20 and thereby actuate the head 16 radially over the disk 4 while writing the servo sectors 20–2N. Rotating the actuator arm 18 about the pivot 20 causes the head 16 to traverse an arcuate path from the outer to inner diameter of the disk 4 which forms substantially arcuate servo wedges on as shown in FIG. 1.
A rotary actuator, such as a voice coil motor, is typically used to actuate the head 16 over the disk 4 during a self-servo writing technique as well as during the disk drive's normal operation. Current is passed through a voice coil comprising a first leg 22A and a second leg 22B attached to the base of the actuator arm 18. The resulting magnetic flux interacts with the magnetic flux of permanent magnets (not shown) creating a torque that rotates the actuator arm 18 about the pivot 20 in order to access a target data track 6 during write and read operations.
Perpendicular magnet recording is a recent development in disk drive technology that provides a significant increase in linear bit density as compared to conventional longitudinal magnetic recording. With perpendicular magnet recording, the head 16 comprises a write element having a leading pole and a trailing write pole, wherein a write coil is wrapped around the write pole. The disk 4 comprises a high permeability magnetic layer underneath a perpendicularly magnetized thin film layer such that an image of the write pole is produced in the high permeability magnetic layer when the write coil is energized with current.
FIGS. 2A–2C illustrate a problem that manifests when servo writing the disk 4 using the head 16 internal to the disk drive with the new perpendicular magnetic recording technique. When writing the servo sectors 20–2N from the inner diameter of the disk (FIG. 2A) toward the outer diameter of the disk (FIG. 2C), the skew angle of the write pole 24 as it approaches the outer diameter causes the inner corner of the write pole 24 to “swing out” and overwrite a band 26 of the previously written servo data. Similarly, when writing the servo sectors 20–2N from the outer diameter of the disk (FIG. 2C) toward the inner diameter of the disk (FIG. 2A), the skew angle of the write pole 24 as it approaches the inner diameter causes the inner corner of the write pole 24 to “swing out” and overwrite a band 28 of the previously written servo data. The overwritten band (26 or 28) creates a “seam” between adjacent servo sectors, as well as a seam within each servo sector (including the servo bursts 14) if multiple revolutions are used to “stitch” together each servo sector 2i. The technique of “stitching” together a servo sector is typically necessary since the width of the write pole 24 is less than the width of a servo track requiring a portion (e.g., half) of a servo sector to be written during each revolution of the disk. The seams created by the overwrite problem illustrated in FIGS. 2A–2C induce errors in the position error signal generated when reading the servo bursts 14 as well as errors in detecting the servo data field 12, such as the Gray coded track addresses.
The prior art has suggested a number of techniques for addressing the overwrite problem when servo writing a disk drive using perpendicular magnetic recording. For example, U.S. Pat. No. 6,504,675 discloses a disk drive wherein the write pole has a trapezoidal shape in order to reduce the overwrite problem caused by the skew effect. However, the geometry of the trapezoidal shape varies between each disk drive due to tolerances in manufacturing the head, resulting in undesirable seams in the servo wedges for some percentage of the disk drives. In addition, manufacturing the write pole with a trapezoidal shape increases the manufacturing cost of the head, as well as reduces the surface area of the write pole leading to an undesirable decrease in the strength of the magnetic write flux.
U.S. Patent Application No. 2004/0061967 suggests an alternative solution to the overwrite problem by writing the servo sectors 20–2N from the outer diameter of the disk to the middle diameter, and then from the inner diameter to the middle diameter. A problem with this technique, however, is the seam created near the middle diameter of the disk where the two segments of a servo wedge “meet”. This seam becomes unusable (wasted) surface area, and the seek operation in the servo system must also account for the seam. This problem is exacerbated due to the disk expanding during the servo writing operation requiring a predetermined margin (wider seam) to account for the worst-case deviation in the expansion.
There is, therefore, a need to servo write the disk of a disk drive in a manner that reduces the seams caused by the write pole overwriting previously written servo data due to the skew effect.